Active optical current measuring system

ABSTRACT

An active optical current-measuring system (2) including a sensor (4) which is provided with current connecting busbars (12, 14) and a light guide connector. The sensor includes two parts (8, 10) which, when assembled, form a hollow cavity (16) within which an electronic sensor component (6) is mounted. The electronic sensor is connected at the output to the light guide connector. A measuring resistor is provided as the sensor (4). The current-measuring system (2) is also capable of detecting direct currents. The electronic sensor component,(6) is mechanically protected and electromagnetically shielded.

FIELD OF THE INVENTION

The present invention relates to an active optical current measuringsystem.

BACKGROUND INVENTION

The intermediate circuit voltages in large-scale power converters havealready reached approximately 5 kV today and tend toward 10 kV and 20kV. Such high intermediate circuit voltages also make high demands ofthe insulation resistance of the current and voltage transformers. Thepartial discharge strength is the most important design parameter forthe insulation resistance. At an intermediate circuit voltage of 5 kV, apartial discharge strength of 20 kV must be required.

The transformer systems in use today are compensating currenttransformers. To maintain the required partial discharge strength,however, these transformers must be manufactured in accordance withtechniques that greatly increase the cost of the transformers.Furthermore, these transformers take up a great deal of space, and thelines that provide power to the transformer and that transmitmeasurement signals must be installed in compliance with specificationsfor the clearance in air and the creepage distance to guarantee reliableisolation. This further increases the amount of space required.

Since transformers can hardly guarantee a partial discharge strength of20 kV at an intermediate circuit voltage of 5 kV, increasingly expensiveinsulation is not a suitable way to make, at a relatively inexpensivecost, a strong transformer system suitable for even higher intermediatecircuit voltages.

A fundamentally different approach toward solving this problem is toachieve reliable isolation by means of light guides. However, this meansthat an electronic component connected to the sensor is needed toamplify at least the measurement signals supplied by the sensor and tocontrol a light guide transmitter. However, the power supply to thiselectronic component must then also be transmitted over a light guide.Such power transmission systems comprise a laser, a light guide, and apower converter. However, the transmissible power is limited to a fewhundred milliwatts. For this reason, no compensating current transformercan be used in systems that are supplied with light energy because thepower demand by these transformers is too great due to the compensatingcurrent.

The article "Optical Current Transformer--Successful First Field TrialIn A 380 kV Network", printed in the German journal ABB Technik, vol. 3(1994) pages 12 through 18, discusses an active optical currentmeasuring system. This article presents various optical currenttransformers that have been tested in a German 380 kV network underactual conditions. The active current transformer functions essentiallyaccording to the conventional current measurement principle,supplemented by a digital optical transmission link. Specifically, thesystem comprises an air-core inductor, including load impedance, ananalog-digital converter, and a transmitter unit with a light-emittingdiode at a high potential. A light guide establishes the connection tothe interface device at ground potential. The electronic component thatis at a high voltage potential has a power demand of less than 150 μW.This electronic component is designed as a low-power, CMOS integratedcomponent. A laser diode that transmits power between two data telegramsover the same light guide is sufficient as the power supply. At greaterdistances between the sensor and the interface device, a parallel lightguide is needed for the power supply because of the higher transmittingpower.

The article "EHV Series Capacitor Banks. A New Approach To Platform ToGround Signaling, Relay Protection And Supervision," published in thejournal IEEE Transactions on Power Delivery, vol. 4, no. 2 (April 1989)pages 1369 through 1378, describes the use of an active currenttransformer with a series compensator. This article describes the designof an active current transformer, an electronic sensor component and aninterface device at ground potential. The electronic sensor componentcomprises a filter, a plurality of voltage dividers, and an energyconverter. The interface device comprises a receiver, a limit valuecircuit, a detector, a buffer, a pulse generator, a transmitter, and alight filter. Since the active current transformer uses an air-coreinductor with a load impedance for the current measurement, onlyalternating currents can be detected with this active currenttransformer.

A measuring resistor or shunt is a current sensor that can also detectdirect currents and does not need a separate power supply. Since themeasurement signals of the shunt must be kept low in the interest ofkeeping the power loss low, an electronic sensor should be located inthe immediate vicinity of the shunt to prevent most interference. Thiselectronic component is then operated in a rough electromagneticenvironment, i.e., it is exposed to strong and often quickly changingelectric and magnetic fields. In addition, the electronic component isusually exposed to relatively high temperatures. One disadvantage of themeasuring resistor is that it must be machined for an adjustment.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an active opticalcurrent measuring system with which direct currents can be measured andwhich comprises an electronic sensor component which is protected frominterference.

The sensor of the system of the present invention comprises two partswhich, when assembled, form a hollow cavity in which the electronicsensor is mounted. A measuring resistor (shunt) is provided as thesensor and forms the casing for the electronic sensor component. Sincethe shunt acts as an electromagnetic shield, this design guaranteesmechanical protection of the electronic component and preventshigh-frequency interference from acting on the electronic component. Theactual sensor signals are picked up on the inside of the shunt.

In an embodiment of the active optical current measuring system of thepresent invention, the electronic sensor component is linked on theinside at the input to two measuring points that are close togetherspatially, with the first measuring point being a point on the inside ofthe sensor, and the second measuring point being connected to anotherpoint on the inside of the sensor by an insulated line running along theinside of the sensor. This additional point is selected so that thesecond measuring point and this additional measuring point are separatedspatially by a relatively great distance. By means of this insulatedline, the potential applied to the additional point is transmitted tothe second measuring point with a low inductance at the shunt. In thisway, the resulting measuring-circuit voltage is always exactly thevoltage corresponding to the voltage drop caused by the current on theclosed conductor loop along the path of the measuring line.

In another embodiment, the second measuring point is connected to theadditional point on the inside of the sensor by means of severalinsulated lines running along the inside of the sensor, which forms acage. With this design of the multiple measuring lines forming a cage,immunity with respect to external interference fields is greatlyincreased.

In another embodiment, the electronic sensor component haselectronically controlled potentiometers. This permits electronicadjustment of offset and gain. Therefore, the shunt can be manufacturedfirst, then the electronic component inserted and the current-measuringsystem adjusted without machining. The respective loop positions of theelectronically controlled potentiometers are stored. When the activeoptical current-measuring system is switched on, the stored position isautomatically read out again and the potentiometers are adjusted.

This current measuring system is adjusted with regard to the zero pointand gain or transfer coefficient. Transmission of the signals foradjusting the electronically controlled potentiometers can beaccomplished over a plug-and-socket connection provided on a shunt andalso by the fact that the light sent by the transmitter of theelectronic analyzer at ground potential is modulated and the informationis recovered at the electronic sensor. The adjustment can be performedfully automatically.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the structural design of a current measuring systemaccording to the present invention.

FIG. 2 shows a block schematic diagram of an electronic sensor componentof the current measuring system according to FIG. 1.

FIG. 3 illustrates a shunt for determining a sensor signal.

FIG. 4 illustrates a compensating cage.

DETAILED DESCRIPTION

FIG. 1 shows the physical design of an active optical current measuringsystem 2 according to the present invention. The system includes asensor 4 and an electronic sensor component 6. Sensor 4 is a measuringresistor (shunt) that consists of parts 8 and 10. Each lateral side ofparts 8 and 10 is provided with a current connecting busbar 12 and 14.In addition, the shunt parts 8 and 10 are designed so that whenassembled, shunt 4 has a hollow space 16.

Electronic sensor component 6 is arranged in the hollow space 16. Shunt4 practically forms the casing for electronic sensor component 6, whichis illustrated in further detail in FIG. 2. The two parts 8 and 10 ofshunt 4 may be made of manganin, whereas current connecting busbars 12and 14 may be made of copper. Current connecting busbars 12 and 14 maybe hard soldered, for example, to parts 10 and 8, respectively. Thesecurrent connecting busbars 12 and 14 provide the connection for the twoshunt parts 8 and 10 on the one hand and on the other hand they providefor the mounting in a busbar, e.g., a busbar in an intermediate circuitof a large-scale power converter. To connect the two shunt parts 8 and10, current connecting busbars 12 and 14 are screwed, riveted, or weldedtogether. Sensor 4 is also provided with two light guide connectors (notshown in this diagram). The two light guides 18 and 20 are plugged intothe light guide connectors, with light guide 18 being responsible fortransmission of the data stream or the complex signal consisting of aPWM signal and a data signal, and light guide 20 being responsible fortransmission of the power for electronic sensor component 6. This designof the active optical current measuring system provides mechanicalprotection of electronic sensor component 6 and prevents high-frequencyinterference from influencing electronic sensor component 6. Thisprotection is due to shunt 4 acting as an electromagnetic shield.

FIG. 2 shows a block schematic diagram of electronic sensor component 6.The electronic sensor component 6 comprises an energy converter 22, ameasurement unit 24, an electronic processing component 26, and adigital mixing unit 28. Measurement unit 24, which serves to regulatethe power supply voltage of electronic processing component 26, has areference value generator 30, a voltage divider 32, a comparator 34, anda pulse-width modulator (PWM) 36. Power in the form of laser lighttravels over light guide 20 to energy converter 22, which converts thislight back into electricity. The voltage built up by energy converter 22serves directly, i.e., without additional in situ voltage regulation, tosupply current to electronic processing component 26, which is linked atthe input to sensor 4.

In order to make it possible nevertheless to keep the power supplyvoltage of electronic processing component 26 constant, this voltage ismeasured and compared with a reference voltage. The difference is pulsewidth modulated and transmitted to the electronic analyzer of theinterface module at ground potential together with the data streamsupplied by electronic processing component 26 with the help of mixingunit 28, in particular a multiplexer. The actual measuring signal andthe PWM signal are separated at this point as well. While the measuringsignal is routed to a processor, the PWM signal is sent as the inputsignal to a controller that varies the power of the transmitter in sucha way that the voltage built up by energy converter 22 is kept constant.

The actual sensor signals are picked up on the inside 38 of shunt 4.Because of the mechanical design of shunt 4, the voltage drop acrossshunt 4 can always be picked off only between two points A and C whichare separated by a relatively great distance (FIG. 3). To be able toprocess the measuring-circuit voltage in an electronic component 6 (notshown in detail in this figure for the sake of simplicity), themeasuring-circuit voltage must, however, be sent to an amplifier havingterminals generally located in close proximity to one another. Thus, themeasuring-circuit voltage must be carried over a line 40. The line 40 islaid close to the inside 38 of shunt 4 and is insulated. The line 40serves to transmit the potential existing at point A to point B with alow inductance on the shunt. Measuring point B is located in closeproximity to measuring point C. The actual sensor signal can be pickedup between the points B and C and provided to electronic sensorcomponent 6.

In order to prevent interference, it is necessary to ensure that no openconductor loop exists between the closed conductor loop (inside 38 ofshunt 4) and measuring line 40. This condition can be met only by aclosed measuring conductor loop that is practically identical to thefirst conductor loop, but is insulated from it. The two conductor loopsare connected to each other only at point A. However, this interferencecompensation is linked to a condition, because an interference fieldpermeating the external conductor loop and inducing a current in it alsopenetrates through the measuring conductor loop and likewise induces aleakage current in this conductor loop. Complete compensation of theexternal interference field results only when the ratio of the partialresistances of the conductor loop and the measuring conductor loop isthe same. Only in this case are the voltage drops caused by the inducedcurrents in the external conductor loop (inside 38 of the shunt) and themeasuring conductor loop the same and cancel out each other.

To obtain a measuring conductor loop, another measuring line 40 must beprovided, that is connected to the first measuring line 40 at point Aand to shunt 4 and the first measuring line 40 at point B (FIG. 4).Since shunt 4 is three-dimensional, the interference fields can act onshunt 4 in any direction and induce corresponding eddy currents.Therefore, the measuring conductor loop must also have athree-dimensional design. Here it assumes the form of a cage 42according to FIG. 4 which conforms to the inside 38 of the shunt, but isinsulated from it. In the ideal case, cage 42 forms a closed casing.With a symmetrical cage 42 consisting of only six individual conductors40, interference compensation of approximately 90% in comparison with asingle conductor loop is achieved. As FIG. 4 indicates, individualconductors 40 of cage 42 run in the direction of the external currentflow. This does not permit compensation of any eddy currents across thedirection of current flow, but they do not in a first approximationcontribute to a falsification of the measuring-circuit voltage of shunt4.

Electronic processing component 26, which comprises an amplifier circuitat the input and an analog-digital converter at the output, is designedto permit electronic offset and gain adjustment. Thus, shunt 4 can bemanufactured first, after which electronic sensor component 6 can beinserted and active optical current-measuring system 2 can be adjustedwithout machining. For electronic adjustment of current-measuring system2, electronically controlled potentiometers are provided in theamplifier circuit of electronic processing component 26. The respectiveloop position is stored at the end of the adjustment. When the device isswitched on, the stored loop position is automatically read out againand the potentiometer is adjusted. The signals for adjusting thepotentiometers can be transmitted over a plug-and-socket connectionprovided on the shunt or by the fact that the light received by energyconverter 22 is modulated and the information is recovered in electronicsensor component 6. The adjustment can be made fully automatically.

What is claimed is:
 1. An active optical current-measuring system,comprising:a first busbar; a second busbar; at least one light guideconnector; a sensor coupled to the first busbar, the second busbar, andthe at least one light guide connector, wherein the sensor comprises afirst sensor part and a second sensor part, the first sensor part andthe second sensor part forming a hollow cavity in the sensor; and anelectronic sensor component mounted within the hollow cavity,the-electronic sensor component having an output coupled to the at leastone light guide connector.
 2. The active optical current measuringsystem according to claim 1, wherein the sensor comprises a measuringresistor.
 3. The active optical current measuring system according toclaim 1, wherein:the electronic sensor component has an input connectedto a first measuring point and to a second measuring point, the firstmeasuring point being located proximate to the second measuring point, ameasuring-circuit voltage is picked up at the first measuring point andthe second measuring point, the first measuring point is located on aninside portion of the sensor, the second measuring point is coupled toan additional point on the-inside portion of the sensor by an insulatedline running along the inside portion of the sensor, and the secondmeasuring point and the additional point are located on substantiallyopposite ends of the sensor.
 4. The active optical current measuringsystem according to claim 3, wherein the second measuring point isconnected to the additional point on the inside portion of the sensor bya plurality of insulated lines running along the inside portion of thesensor.
 5. The active optical current measuring system according toclaim 4, wherein the plurality of insulated lines runs in a direction ofa sensor current, and wherein the plurality of insulated lines forms acage.
 6. The active optical current measuring system according to claim5, wherein the cage forms a closed casing.
 7. The active optical currentmeasuring system according to claim 1, wherein the electronic sensorcomponent includes a plurality of electronically controlledpotentiometers.
 8. The active optical current measuring system accordingto claim 1, wherein each one of the first sensor part and the secondsensor part is coupled to at least one of the first busbar and thesecond busbar.
 9. The active optical current measuring system accordingto claim 1, wherein the sensor comprises manganin.
 10. The activeoptical current measuring system according to claim 1, wherein each oneof the first busbar and the second busbar comprises copper.
 11. Theactive optical measuring system according to claim 1, comprising atleast two light guide connectors.
 12. The active optical currentmeasuring system according to claim 1, wherein the sensor forms anelectromagnetic shield for the electronic sensor component.
 13. Theactive optical current measuring system according to claim 1, whereinthe sensor mechanically protects the electronic sensor component. 14.The active optical current measuring system according to claim 1,wherein the electronic sensor component includes a measurement unit. 15.The active optical current measuring system according to claim 1,wherein the electronic sensor component includes an energy converter.16. The active optical current measuring system according to claim 1,wherein the electronic sensor component includes an electronic processorcomponent.
 17. The active optical current measuring system according toclaim 1, wherein the electronic sensor component includes a pulse-widthmodulator.
 18. The active optical current measuring system according toclaim 1, wherein the electronic sensor component includes anelectro-optical system.